Radio receiver muting circuit



11. AMPL. Dsrscm Dec. 14,1943. E. o. SELBY 2,337,005

RADIO RECEIVER MUTING CIRCUIT Filed April 4, 1942 SOURCE LOCAL OSCILLATOR INVENTO R Patented Dec. 14, 1943 RADIO RECEIVER MUTING CIRCUIT Eugene 0. Selby, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 4, 1942, Serial No. 437,613

6 Claims.

My present invention relates to muting circuits in radio receivers, and more particularly to a carrier-operated circuit adapted for inclusion in a radio receiver for the purpose of suppressing receiver background noises, or received external noise disturbances, Whendesired carrier energy of less than a predetermined amplitude is being collected at the receiver.

One of the main objects of my present invention may be stated to reside in the provision of an automatic muting, or noise squelching, circuit which offers advantages over existing circuits of the same general type in that it can be included in an existing radio receiver system without afiecting the receiving system performance appreciably, and the muting circuit being adjustable over a wide range of carrier inputs while maintaining a sharp cut-off characteristic over the complete range of adjustment.

Another important object of this invention is to provide an automatic noise suppression circuit which is responsive to the disappearance of received carrier energy, and the suppression circuit functioning to prevent the transmission of signal energy between the demodulator output circuit and the input electrode of the modulation voltage amplifier.

Another object of the invention is to provide a carrier-operated anti-noise device which consists of a diode inserted in the direct audio signal transmission path to the input grid of the first audio frequency amplifier, and the conductivity of the diode being controlled by an electron discharge tube Whose input electrode has its potential varied in response to carrier-derived control potential.

Still other objects of my invention are to improve generally the simplicity and efficiency of automatic noise suppression systems, and more particularly to provide a receiver muting circuit which is not only reliable in operation, but is economically manufactured and assembled in existing radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the followingdescription taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into eifect.

' Referring now to the accompanying drawing, there are shown the various networks preceding the demodulator circuit. These pre-demodulator networks are schematically represented, since those skilled in the art are fully acquainted with the circuit details. While the invention is applicable to any type of radio receiver, the latter is shown as of the superheterodyne type. Generally, it comprises a radio frequency amplifier Whose input terminals may be coupled to any desired modulated carrier wave source. The amplifier may comprise one or more stages of tunable radio frequency amplification. The amplified modulated carrier Waves are then fed to the tunable input circuit of a first detector. The latter is fed with local oscillations from a local oscillator. The local oscillator is .tunable over a range of frequencies which differs constantly from the signal range by the value of the operating intermediate frequency (I. F.). The I. F.- output of the first detector is fed to an I. F. amplifier, and the latter may comprise one or more stages of amplification. Each of the latter stages is fixedly tuned to the I. F. value. The demodulator, or second detector, is shown as a diode device Whose anode is connected to the high potential side of the tuned secondary .circuit of the I. F. output transformer T. As those skilled in the art fully know, the primary and secondary circuits of the transformer T are each tuned to the I. F. value.

The low potential side of the secondary circuit of transformer 'I is connected to ground through load resistor I. The upper end of resistor l is by-passed to ground for I. F. currents by condenser 3. The cathode of the diode demodulator returns directly to ground. I he direct current voltage component of the rectified I. F. current flowing through the diode load resistor is utilized usually for automatic volume controlv (AVC) of the various stages preceding the demodulator. Such an AVC circuit is schematically designated fAVC in the drawing, and it will be understood that the AVG bias is applied 'to the signal grids ,of the radio frequency amplifiers and the signal grids of the I. F. amplifiers. The magnitude of the direct current voltage developed across :the diodeload resistor depends upon the magnitude of the carrier received. The AVC circuit, as is 'well known, functions to control the gain of the radio frequency and .I. :F. amplifiers so ,as :to maintain the carrier' amplitude at transformer T substantially uniform .over a wide range ofv variations of the received signal ,en'ergyat the receiver antenna. I

'The modulation voltage component of the rectified I. F. current is applied to one or more stages of audio frequency amplification. first audio frequency amplifier tube is designated by numeral I8, and its input grid is shown connected to the load resistor through a path which comprises the adjustable potentiometer tap 2, condenser I4, the cathode to anode path of a diode I! and condenser I6. The cathode of amplifier I8 includesin circuit therewith the usual grid biasing network I9, the control grid of tube I8 being returned to ground by the re-jf V voltage drop across resistor I I is due to the plate sistor I5. The modulation, or audio, voltage output of tube l8 may be transmitted to subsequent audio frequency amplifier stages; The diode I1 is inserted in .the pathto the input electrode of the audio frequency amplifier in order to provide background noise suppression below a when the received carrier energy falls predetermined amplitude.

As is well known in the art of radio receivers,

when lAVC is employed the gain of each controlled amplifier'is a maximum when the received carrier'energy has a small amplitude, or when no modulated carrier waves are being received. For this reason, the tube noises, or noises picked up from external sources, are greatly amplified and are 'reproduced 'in the receiver loudspeakers Hence, when tuning between station settings of the receiver, or in cases when the received carrier amplitude falls below a predetermined threshold value, the noisereproduction is intolerable; -'Noise muting circuits are, therefore,

The

- voltage of the audio signal at the input grid of provided to prevent noise reproduction in such cases.

My present background noise suppressor circuit is readily applied to any existing receiver,

since it is only necessary toinsert the diode I! in the audio input'path from the demodulator load resistor to the input grid of the first audio amplifier. The conductivity of the diode is regulated directly from'the demodulator diode load resistor by means of a tube 20. Where AVC is used, it is only necessary to utilize some of the AVG bias to regulate the action of tube 20. The tube 20 maybe a pentode tube of the 6SK7- type. The diode I! may be one'diode section of a double diode of the 6H6 type. Of course, these two tube types are purely illustrative, and other suitable types of tubes may readily be substituted.

The control-grid 2| of tube 20 is connected to the junction of a resistor 5 and condenser 6 providing a time 'delay network. The lower terminal of condenser 6 is grounded, while the upper end of resistor 5 is connected to the anode end of resistor I. The cathode of tube 20 is connected to ground through a resistor I. The plate 22 is connected to junction A of series resistors I2 and II. P A condenser 24 connects from point A to ground. The upper end of resistor I2 is connected to the anode of diode II, while the lower end of resistor II is connected to one end of potential supplypotentiometer ID. The direct current potential supply potentiometer is not shown in its entirety, but it will be.understood that the sections 9 and I0 thereof may be a part of the main voltage supply network of the receiver. One end of resistor 9 is grounded, while the junction B of sections 9 and I0 is connected through resistor l3to the cathode of diode II. The right hand end of resistor I0 is indicated as having a-potential, byway of example, of plus 200 volts with respect to ground. l

The screen grid 23 of tube 20 is arranged for adjustable connection to the resistor section 9 through a path which comprises the voltag rethe audio amplifier I8. The actual bias on the noise control diode I1 is somewhat less than the voltage drop across resistor I0 by virtue of the voltage drop across resistors II, I2 and I3. The

current of tube 20, and the current fiow from the diode itself through the resistors I2 and I3.

It will be noted in this connection that the diode II has its cathode to anode path arranged in series in the circuit which comprises resistors I2,

II, II] andIB."

The plate current of the tube 20 is controlled directly from the received carrier. Tube 20 functions as a direct current amplifier inthe muting circuit. The control grid 2'I of tube 20 is biased by the direct current voltage derived from the received carrier energy across the second detector loadresistor I. For a given carrier input level the plate current of tube 20 is adjusted by means of the screen voltage adjustment along the potentiometer section 9. The screen voltage when the received carrier amplitude falls below.

this threshold value, the negative bias on the. grid 2I of-tube 20 is decreased by virtue of the reduction in the received carrier amplitude. This, of course, results in an immediate increase of the plate current flow of tube 29. As a result, the potential drop between points A and B reverses from its previous condition, and causes the diode I! to be cut off. Since the diode I! is rendered r non-conductive, the efiect is to provide an openv circuit between the demodulator load resistor and the input electrode of theaudio frequency amplifier tube.- As a result the reproduction of undesired noise impulses at the receiver reproducer is effectively prevented. The anode of the diode I'I. isnormally positive relative to the cathode. The anode becomes negative when the plate current of tube 20 is large enough to cause a potential difference across resistor II which exceeds the drop across resistor I0.

Merely by way of illustration, and in, no. way limiting, there is given herewith ailist of constants which can be employed in the circuit disclosed herein:

' R1 equals 0.54'megohm R5 equals 0.27 megohm R13-R12R15 equals 2.2 megohms R11 equals 0.47 megohm Rs equals 0.22 megohm Rs equals 0.1 megohm R10 equals 22,000 ohms R1 equals 3,900 ohms C3 equals m'mf.

Ce equals 0.01 mfd. Cie-e-Cm equals 0.01 mfd. C24 equals 0.25 mfd.

It is pointed out that the networks 5-6 and II-i-24 .introduce a slight time delay between application of a signal and theclosure of the audio input. Hence, noise surges of short duration will not cause the diode I! to become conductive and pass the noise surges to the audio system.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that m invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a radio receiving system of the type including a detector provided with a signal input circuit and means for utilizing detected. signal energy, an automatic noise muting circuit which comprises a first electronic device of unidirectional conductivity, said device being the sole coupling element connected directly in the detected signal path between said detector and said utilization means, potential supply means normally maintaining said device conductive to pass detected signal energy, a second electronic device having an input electrode and an output electrode, said second device being independent of said utilizing means and functioning solely as a direct current voltage responsive device, means responsive to a decrease in received signal energy below a predetermined amplitude for changing the direct current voltage of said input electrode for increasing the output electrode current flow of said second device, and means, responsive to said increased output electrode current flow, for rendering said first device non-conductive.

2. In a radio receiver provided with a demodulator having a load resistor across which is developed demodulated signals and direct current potential, a modulation signal amplifier tube having a signal input electrode, a diode directly coupling said demodulator load resistor to said input electrode and being the sole demodulation signal transmission path, means normally rendering said diode conductive thereby to permit said diode to transmit demodulated signals to said input electrode, an electron discharge device responsive to solely direct current voltage and having input and output electrodes and being independent of said amplifier tube, means controlling the potential of the last input electrode with solely the aforesaid direct current potential, and means connecting said output electrode to said diode in such a manner that an increase of current fiow through said device above a predetermined value renders said diode non-conductive.

3. In a radio receiving system of the type including a detector provided with a signal input circuit and audio amplifier means for utilizing detected signal energy, an automatic noise muting circuit which comprises a diode inserted directly in the path between said detector and said utilization means, said path being the sole detected signal energy transmission path, means normally maintaining said device conductive to pass detected signal energy, a direct current voltage amplifier tube having an input electrode and an output electrode and independent of said audio amplifier means, means responsive to a decrease in received signal energy below a predetermined amplitude for changing the direct current voltage of the input electrode for increasing the output electrode current flow of said tube, and means, responsive to said increased output electrode current flow, for rendering said diode non-conductive.

4. In combination with a demodulator provided with a signal input circuit and means for utilizing detected signal energy, a first device of unidirectional conductivity acting as the sole path between said demodulator and said utilization means, potential supply means normally maintaining said device conductive to pass detected signal energy, an electron discharge device independent of said utilizing means having an input electrode and an output electrode and acting as solely a direct current voltage amplifier, means responsive to a decrease in received signal energy below a predetermined amplitude for changing the direct current voltage of said input electrode in a positive polarity sense for increasing the output electrode current flow of said electron device, means, responsive to said increased output electrode current fiow, for rendering said first device non-conductive, and additional means for preventing noise surges of short duration from rendering said first device conductive.

5. In a radio receiving system of the type including a detector provided with a signal input circuit and means for utilizing detected signal energy, an automatic noise muting circuit which comprises a device of uni-directional conductivity, said device being normally conductive and being inserted directly in the path between said detector and said utilization means, an electron discharge tube having an input electrode, a positive screen grid and an output electrode, means responsive to a decrease in received signal energy below a predetermined amplitude for increasing the output electrode current flow of said tube, means, responsive to said increased output electrode current flow, for rendering said device non-conductive, and means for adjusting the positive potential of said screen grid over a wide range of potential values.

6. In a radio receiver provided with a demodulator having a load resistor across which are developed modulation signals and direct current potential, a modulation amplifier tube having a signal input electrode, a diode coupling said demodulator load resistor to said input electrode, potential supply means normally rendering said diode conductive thereby to permit said diode to transmit modulation signal to said input electrode, a resistive path between the anode of the diode and the positive terminal of the potential supply, a resistor connected between the diode cathode and the low end of said potential supply, an electron discharge tube having input and output electrodes, means controlling the potential of the last input electrode with the aforesaid direct current potential, and means connecting said last tube output electrode to an intermediate point of said resistive path such that an increase of current fiow through said last tube above a predetermined value renders said diode non-conductive.

EUGENE O. SELBY. 

